1. Field of the Invention
The present invention relates to a super-absorbent resin. In particular, it relates to a super-absorbent resin used in such as a disposable diaper or a sanitary napkin, characterized in absorbing body fluid but being soluble in water.
2. Description of the Related Art
At present, in hygienic goods such as a disposable diaper or a sanitary napkin, so-called an incontinence pad, as component material thereof, a super-absorbent resin or hydrophilic fiber such as pulp has been widely used aiming at absorbing body fluid. As the super-absorbent resin, for example, partially neutralized and crosslinked polyacrylic acid, a hydrolyzed or crosslinked starch-acrylonitrile graft polymer, a neutralized or crosslinked starch-acrylic acid graft polymer, a saponified or crosslinked vinyl acetate-acrylate ester copolymer, a hydrolyzed or crosslinked acrylonitrile copolymer, or a crosslinked acrylamide copolymer and a crosslinked polymer of a cationic monomer have been used as a main raw material.
As a method for improving performance of the conventional super-absorbent resin, methods for attaining high absorption capacity to aqueous fluid such as body fluid excellent absorption rate, and high fluid permeability have been proposed in U.S. Pat. No. 5,601,542 and U.S. Pat. No. 5,669,894. However, super-absorbent resins which have been proposed up to now, because they are prepared in a firmly cross-linked form by using a cross-linking agent and the like, not only exhibit high absorption to an aqueous solution containing a salt such as body fluid, but also exhibit high absorption capacity to water (including ion-exchanged water, distilled water, and tap water), and had a problem that a drainage pipe could be clogged when they are flown into a flushing toilet and the like. Due to this problem, the conventional super-absorbent resins have actually been necessary to be disposed by incineration, landfill, and the like, which necessitates a large amount of energy.
In addition, according to a well-known technology, so-called an ion-sensitive film or binder has been proposed, solubility of which is varied depending on ion concentrations (see U.S. Pat. No. 5,631,317, U.S. Pat. No. 5,317,063, U.S. Pat. No. 5,312,883, U.S. Pat. No. 4,535,098, U.S. Pat. No. 6,291,372, U.S. Pat. No. 6,423,804, U.S. Pat. No. 6,683,143, U.S. Pat. No. 6,537,663 and JP-A63-139906, for example).
However, the technologies disclosed in these publications have such a problem as of low absorption performance to body fluid, and low solubility in water, and have failed to provide such an absorbent agent of body fluid that can be flushed down a toilet basin. Further, in the use thereof, a method for producing an absorbing core with high strength has been adopted, which comprises spraying a polymer solution as a binder on an absorbing core web formed, and drying. There, however, has a problem that an absorbing core becomes hard and has decreased acquisition rate of fluid into an absorbing core, when a large quantity of the polymer was introduced in an absorbing core. Accordingly, such a conventional polymer could not be used except for as a small amount of an additive like a binder, and water-absorbing performance has not been pursued to the polymer.